1. Field of the Invention
This invention relates in general to techniques for measuring the characteristics of thin films. In particular, it relates to white-light scanning interferometric methods and systems for measuring the thickness of transparent thin-film coatings.
2. Description of the Related Art
White-light scanning interferometers (WLSIs) have gained popularity in opto-electronic industry for testing that goes beyond measuring the thickness of various elements of opto-electronic circuitry. The measurement technique of white-light scanning interferometry produces a short coherence interferogram that enables the measurement of the optical path difference between test and reference beams without 2π ambiguity. Unlike other conventional techniques, such as ellipsometry or spectroscopy, which cannot measure surface profile, this interferometric technique can produce both thickness and surface profile measurements of a transparent object under test, thus effectively providing the user with information about its three-dimensional profile.
As is well understood in the art, a WLSI utilizes interferometric signals (often referred to as correlograms) generated by changing the length of the sample arm of the interferometer with respect to its reference arm in minute incremental steps (typically, several nanometers long) within the coherence length of the light utilized for the measurement. The interferometric signal produced by an uncoated substrate (whether flat or patterned) clearly differs from the signal produced by a substrate coated with a thin film because of the multiple reflections generated within the film. Accordingly, these two signals can be used to determine the thin-film thickness distribution. U.S. Pat. No. 6,545,763 by Kim et al. describes a method for extracting thin-film thickness and profile data from the Fourier transform of the interferometric correlogram produced by a substrate coated with a thin film. The approach makes use of the dependency of the phase component of the Fourier transform on film thickness. The spectral phase information is compared to the phase information obtained by modeling the measurement and an error function is generated and minimized. This approach has shown not to provide repeatable results when the thickness of the film is reduced to about a tenth of a micron or below.
Since the fabrication of dielectric coatings with sub-micron thickness (i.e., below the white-light coherence length of about several microns) is quite common and becomes even more demanding with advances in short-wavelength optical technologies, there remains a need for developing a technique for measuring accurately and repeatably the parameters of dielectric films with a thickness on the order of 100 nm or less. This problem is solved in the present invention by utilizing the amplitude components of the Fourier transform (“Fourier amplitude”) of WLSI correlograms. The approach also produces information about the three-dimensional (volumetric) profile of the thin film and may be extended to provide information about the film's index of refraction.